1. Technical Field
The disclosure relates to a hybrid vehicle. In particular, the disclosure is concerned with a hybrid vehicle including a power transmission system in which an electric continuously variable transmission and a mechanical stepwise variable transmission are connected in series.
2. Description of Related Art
A hybrid vehicle including a power transmission system in which an electric continuously variable transmission and a mechanical stepwise variable transmission are connected in series is known, as disclosed in Japanese Patent Application Publication No. 2006-321392 (JP 2006-321392 A). The electric continuously variable transmission can steplessly change the rotational speed of a drive source (an engine) through torque control of a differential rotating machine (a first motor-generator), and transmit torque from the drive source to an intermediate transmission member as an output-side member of the electric continuously variable transmission. The mechanical stepwise variable transmission is disposed between the intermediate transmission member and drive wheels, and can mechanically establish a plurality of gear positions (which may be called “mechanical gear positions”) having different ratios (speed ratios) of the rotational speed of the intermediate transmission member to the output rotational speed. The mechanical stepwise variable transmission includes two or more frictional engagement elements, and switches power transmission paths by switching engagement and release of selected ones of these frictional engagement elements, so as to establish a given mechanical gear position.
According to a technology described in JP 2006-321392 A, in order to curb occurrence of shift shock due to change of the rotational speed in the inertia phase, during shifting of the mechanical stepwise variable transmission, the speed ratio of the electric continuously variable transmission is changed while the rotational speed of the drive source is kept substantially constant, so that the inertia phase of the mechanical stepwise variable transmission is started.